A common requirement for an advanced electronic circuit and particularly for circuits manufactured as integrated circuits (“ICs”) in semiconductor processes is the use of bandgap reference circuits. Bandgap reference circuits provide a current, or voltage, reference that is ideally temperature and process variation independent. A bandgap reference is designed to have a zero temperature coefficient (“TC”). Bandgap reference circuits are an essential component in many analog and mixed signal circuits where a fixed voltage reference or current reference is required. In order to ensure a highly accurate reference from a bandgap reference circuit in integrated circuit devices fabricated in semiconductor processes, test measurements and device trimming procedures are usually performed. The trimming steps are typically done while the devices are still in the form of dies on a semiconductor wafer such as at the chip probe (“CP”) or final test (“FT”) stages. The trimming is performed to reduce the absolute value error of the reference output, whether voltage or current, due to process variations and first order temperature drift effects. These trimming steps add costs to manufacturing, and add additional testing costs and time to the production of the devices.
Typically, to fabricate a temperature independent circuit, an output that is predictably proportional to absolute temperature (“PTAT”) is combined with an output that is complimentary to absolute temperature (“CTAT”). In this manner the output of the circuit is compensated for temperature drift and ideally would provide a reference current, typically, that is temperature independent. An output current can then easily be used to form a reference voltage output that is also temperature independent. However, practical devices remain subject to temperature drift errors and process variations, and thus, trimming has been used to remove any remaining errors. Trimming typically involves laser trimming. This trimming can be used to adjust impedance values in the circuit to compensate for temperature dependent and process dependent errors measured in the bandgap circuit output. However, the use of trimming techniques requires additional pads, which also reduces the available silicon area, and as described above, adds steps and adds costs to the manufacturing process.
A continuing need thus exists for a bandgap reference circuit that has an output that is temperature and process independent for a wide range of expected conditions without the need for trimming. The bandgap reference should be compatible with existing semiconductor processes and circuits.
The drawings, schematics and diagrams are illustrative and not intended to be limiting, but are examples of embodiments of the invention, are simplified for explanatory purposes, and are not drawn to scale.